Lignin is a complex biopolymer found in the cell walls of plants and is most commonly derived from wood and some algae. Lignin, cellulose, hemicellulose and pectin are the major components in the cell wall and lignin, together with the hemicellulose, provide mechanical strength to the cell wall.
Today lignin may be used as a component in for example pellet fuel as a binder but it may also be used as an energy source due to its high energy content. Lignin has higher energy content than cellulose or hemicelluloses and one gram of lignin has on average 2.27 KJ, which is 30% more than the energy content of cellulosic carbohydrate. The energy content of lignin is similar to that of coal. Today, due to its fuel value lignin removed using the kraft process, sulphate process, in a pulp or paper mill, is usually burned in order to provide energy to run the production process and to recover the chemicals from the cooking liquor.
There are several ways of separating lignin from black or red liquor obtained after separating the cellulose fibres in the kraft or sulphite process respectively, during the production processes. One of the most common strategies is ultra-filtration. Lignoboost® is a separation process developed by Innventia AB and the process has been shown to increase the lignin yield using less sulphuric acid. In the Lignoboost® process, black liquor from the production processes is taken and the lignin is precipitated through the addition and reaction with acid, usually carbon dioxide (CO2), and the lignin is then filtered off. The lignin filter cake is then re-dispersed and acidified, usually using sulphuric acid, and the obtained slurry is then filtered and washed using displacement washing. The lignin is usually then dried and pulverized in order to make it suitable for lime kiln burners or before pelletizing it into pellet fuel. Biofuel, such as biogasoline and biodiesel, is a fuel in which the energy is mainly derived from biomass material or gases such as wood, corn, sugarcane, animal fat, vegetable oils and so on. However the biofuel industries are struggling with issues like food vs fuel debate, efficiency and the general supply of raw material. At the same time the pulp or paper making industries produces huge amounts of lignin which is often, as described above, only burned in the mill.
Two common strategies for exploring biomass as a fuel or fuel component are to use pyrolysis oils or hydrogenated lignin. However these strategies demand high pressure and the use of hydrogen gas which is both expensive and may be dangerous. There is therefore a need for a less complex method to prepare biomass in order for it to be processed at a biorefinery, but also in order to process the lignin into fine chemicals or paint or paint additives for example.